Azo pigments are synthesized by the coupling reaction of a diazonium compound and a coupler. Among the couplers used in the coupling reaction, a particularly important coupler is 2-hydroxynaphthalene-3-carboxylic acid or a derivative thereof.
Brilliant Carmine 6B (Pigment Red 57) and Watchung Red (Pigment Red 48) which are prepared from 2-hydroxynaphthalene-3-carboxylic acid are most important red azolake pigments. These are used in the form of an insoluble metallic lake such as Ca, Ba, Mn, etc., but is inherently unstable to an acid or alkali because of metallic salts.
On the other hand, an insoluble azo pigment is synthesized using, as a coupler, 2-hydroxynaphthalene-3-anilide synthesized by the condensation of 2-hydroxynaphthalene-3-carboxylic acid and anilines. This so-called naphthol pigment has been developed for the purpose of improving the solubility and fastness, and is generally superior to an azolake pigment in properties such as light resistance, weathering resistance, chemical resistance, solvent resistance, etc.
Furthermore, an azo pigment derived from 2-hydroxynaphthalene-6-carboxylic acid which is an isomer of 2-hydroxynaphthalene-3-carboxylic acid, and characteristics thereof are known (Japanese Patent Kokai Publication No. Hei 2-302471).